Michael Cera's acting like a real jerk these days. He's the guy who made his name in 2007 as a superdweeb in Superbad, hapless yet endlessly endearing, and subsequently mined similar territory in Juno and Scott Pilgrim Vs The World. Today, that awkward dork is all but dead and buried. In last year's apocalypse comedy This Is The End he presented a 180-degree subversion of his public image: playing himself, the real Michael Cera was apparently an obnoxious irritant who blew coke in people's faces. In his next two films he is equally insufferable. He also wants to show us his vagina. But more on that later.

In Sebastián Silva's new film (full title: Crystal Fairy & The Magical Cactus), Cera is Jamie, an American in Chile, a drug bore on a quest to find a San Pedro cactus and trip out on the mescaline contained within. It's a role that came out of the blue. Having loved Silva's 2009 drama The Maid, Cera sought out the director and they hit it off. Silva sent him a script for a film called Magic Magic and Cera signed up, moving to Chile for a few months and living with Silva's family. As Magic Magic's funding dragged, Silva decided there and then to make a film about an experience he'd had years earlier. With just a 12-page outline, a handful of actors and a crew of six, they hopped in a van and went north, shooting on the fly and improvising dialogue.

Crystal Fairy fizzes with spontaneity and energy, and thanks to Cera, Jamie is a wonderfully nuanced pig. At a party he meets a good-hearted but overbearing hippy chick who calls herself Crystal Fairy, invites her to join him and his buddies on their "psychoactive voyage", and then immediately regrets doing so. "The thing I brought to it was him doing drugs for this very superficial reason, like a collector," says a perky Cera down the line from Toronto, where he's home for the holidays. "He likes to brag about the drugs he's done, to be an academic about it. I know people like that. That's why it occurred to me that it would be fun to play, because it's annoying, and it's real."

Cera likens his Crystal Fairy experience to "a road trip with friends", the lines blurring between real life and film-making. They shared two tiny cabins on the beach, piled atop of each other at night. "Everything was very intimate," he says. "We were really living it. The movie is more or less a document of this trip we were all going on. I didn't want it to end. I love watching it because of the memories." The film is infused with reality, from the locals playing themselves (including Gypsies and transvestite prostitutes) to the dogs rutting on the street and the consumption of the San Pedro juice. When Cera drinks it on camera, he's really drinking it.

"Sebastián suggested it very optionally: 'We could maybe do this, see what happens,'" he says. "He said: 'If everyone gets uncomfortable we can turn the camera off then the next day we can get back to work.' So it was very gentle. I was all for it. And it wasn't like the whole crew were doing it, so we wouldn't have a day where everyone's freaking out, it was just a way to see if anything interesting happened on camera. A few days leading up to it I was starting to have a little anxiety about just having to work, I didn't know what the effect was going to be. But we didn't actually feel anything. It just felt like we'd had a glass of wine. And we had such a rhythm shooting at that point, it was fine." How did it taste? "Terrible. It's a really acrid, deep bitterness that fills your whole mouth. Earthy. Almost like a wheatgrass shot but much less sweet and a little thicker, swampier."

Gaby Hoffmann, who plays Crystal Fairy, double-dipped and indeed underwent a 10-hour psychoactive voyage; she has described it as "incredibly pleasant", and spent much of the day intently caressing rocks. She drank from a different batch, although Silva has said she finds it easier to let go, and that Cera is more of a control freak. "That's possible," says Cera. "I do think our batch didn't work because it was the same for Sebastián's brothers [who co-star], nobody seemed to feel it. But yeah, I probably was trying to stay in control."

In Magic Magic (out here in April), meanwhile, Cera excels as a spiteful, creepy, closeted homosexual. There was no concerted effort to change his image, he says, but deciding to go to Chile to work with Silva was important to him; he speaks of the experience, the films and the country very fondly. "I only found out he was typecast when we started doing [US] press," says Silva, who had only seen Cera in Juno. "People were so impressed that these roles were so different from everything else he's done. But it was never something we did on purpose. I wasn't trying to break him out of anything, but I was definitely a little shocked by how little of his range had been used. He had played this coy, nerdy character repeatedly, but that's not his fault, it's something that the industry in America tends to do, they're really good at typecasting people. Michael has an acute, ironic sense of humour. He's funny in a dark way."

Cera and co-star Gaby Hoffmann

Cera is increasingly gravitating towards odder material, from the short films he stars in (and often writes and directs) for YouTube comedy collective Jash, to the recent New Yorker piece he penned, playing a yet more deluded, vain version of himself, at one point specifically addressing his typecasting. In the piece, as he forms a wonky relationship with someone who's been texting him in error, he begins anonymously, testing the water by inviting the texter to a party at his "buddy Michael Cera's house". "He's cool," says the texter, who later also confuses him with Jesse Eisenberg. "Really?" replies Cera. "You don't think his range is limited?"

Despite the dorkishness, there was always an otherness to Cera. Says Silva: "I love the way he looks. His eyebrows, his smile, his physicality. It's kind of unusual and mysterious. Imagine how he's gonna look when he's 40. I can't wait to keep collaborating with him." Indeed, the pair plan to work together again soon, possibly for a film in which Cera plays a Hasidic Jew with no friends, and less possibly for a play called Anne The Lesbian.

"That's really our dream," says Silva. "It's kind of a joke, but we make things out of jokes. I think Michael would play a really amazing woman. He would be a super-believable lesbian and I would like to make it even more believable by showing his vagina. We don't know what it's going to be, but that's an idea."

"That's the only idea," says Cera. "If it's a five-minute play it would be great. If it's just a shower scene and I had a prosthetic vagina."

Looking forward to it.

Crystal Fairy is in UK cinemas on 17 Jan, available to download and on-demand from 24 Jan, and out on DVD on 27 Jan